Morning Glory Leaves Wilting and Dying?

The morning glories I had were doing so well, but out of the 4 plants, one of them had it's leaves wilt and dry up. Now another one is doing the same thing. These are well watered plants, even though they are in close proximity. I figured it was maybe the weather since it was starting to get cold at night, but now I am not so sure. The wilting itself seems to be over a period of time, then the actual vine itself dies off.